In the prior art, constant mass, variable-displacement hydrometers have been made of glass. These hydrometers typically comprise a body having a lower ballast section, which usually has a weight secured therein for weighting of the hydrometer, and a stem portion that is integrally formed with the ballast. Typically a paper scale is placed within the stem for providing a direct readout of density. These hydrometers are used by simply placing them in the liquid in which it is to be used, and depending upon the amount which the hydrometer extends into the liquid, an appropriate density can be read off the stem of the device. In these type devices it is important that the outer surface be smooth, transparent, and free of bubbles, striae, or other imperfections that might interfere with the use of the hydrometer. Additionally, these hydrometers should be made of a material that will not react with the chemicals in which it is to be used or exposed, and also have suitable thermal properties to permit its use over a wide range of temperatures. Further, it is also important in these type hydrometers that there be no uneven or unnecessary thickness of the walls, and no abrupt changes or constrictions that would hinder cleaning or tend to trap air bubbles when the instrument is immerged. Because of these high constraints, hydrometers of the prior art have been made of glass. However, these hydrometers, since they are made of glass, are highly subject to breakage.
The present invention concerns an improved plastic hydrometer and method of making same which can provide the stringent requirements required of a constant mass, variable-displacement type hydrometer which is considerably less subject to breakage.